A variety of forms of parallelogram type linkage devices are disclosed for lifting material. Ackermann, U.S. Pat. No. 3,018,011, discloses a fork lift truck having a parallelogram type linkage connected to the forks. A lift ram is connected intermediate the lower arm of each link to effect raising and lowering of the linkage. The upper arm of the linkage is formed by a hydraulic ram which controls tilting of the forks. This arrangement may be satisfactory for lifting light loads. However, with heavy loads such as in the range of 40 tons, extreme bending moments would develop in the lower arm due to the lift ram being pivotally connected intermediate the lower arm. The geometry of the arrangement, resulting from the lift ram being connected to the lower arms of the linkage, requires considerable extension of the lift ram to elevate the forks. In order to minimize sway of the lifted article, plates are provided on each side of the vertical elements for the frames to contain the vertical elements and prevent sway of the load. This places additional side bending moments on the vertical arms which could cause undue stresses in the frame if used to lift high tonnage loads.
Carter, U.S. Pat. No. 2,391,224, discloses a parallelogram type linkage for a tractor loader. A frame is connected to the top of this linkage to control lifting of the linkage. The lift cylinder for the frame is connected to the linkage and to the lifting frame. Although this arrangement may be suitable for low weight farm tractor applications, due to the geometry of the lifting frame as connected to the parallelogram linkage for the fork results in extreme bending moments in the parallelogram linkage and lifting frame. Furthermore, with the lifting frame atop the parallelogram linkage, visibility forwardly of the tractor is hampered.
U.S. Pat. No. 2,656,058 discloses a type of parallelogram linkage for lifting loads onto and off of a truck. The frame consists of a single set of upper and lower arms forward of the frame. The lifting ram is positioned in a manner on the linkage to necessitate considerable extension and swing angle of the ram in lifting the forks. Such an arrangement, although suitable for light loads in lifting materials onto and off the truck, is not acceptable for lifting high tonnage loads.
U.S. Pat. No. 4,054,185 and 4,249,854 disclose linkage mechanisms for lifting heavy loads. However in each arrangement, the lift cylinders are interconnected to the linkage mechanism in a manner which induces bending moments in the arms of the linkage when lifting heavy loads. U.S. Pat. No. 2,665,017 discloses a complex arrangement involving parallelogram linkage where the lift ram is interconnected to the arms of the parallelogram linkage. Due to its construction, extreme bending moments in the lifting arms result if the tractor loader is used to lift and manoeuvre high tonnage objects.
Although the prior art patents have contemplated in many forms the use of parallelogram linkages in lifting loads, no consideration has been given to the use of a parallelogram linkage in lifting high tonnage loads.